a. Field of the Invention
This invention relates to vessels for use in icecovered waters, such as, for example, icebreaking ships, and to novel procedures for breaking ice. More particularly, it pertains to an improved icebreaking vessel equipped with a plow-type bow and a means for supplying an impulse force downwardly from the bow.
B. Description of the Prior Art
Icebreakers traditionally break ice in two alternative ways, namely: either by plowing continuously through the ice sheet relying on the downward force applied by a specially configured, highly raked bow structured to break the ice; or by a technique known as "boxing" or "ramming."
In plowing, a specially configured highly raked bow structure acts like a plow blade that runs under the ice sheet. The displacement of the vessel is that the bow runs under the ice sheet and the vessel is thus displaced downwardly. A moment is presented to the underside of the ice sheet. When the moment becomes sufficient to cause rupture of the ice, complete failure of the ice sheet occurs. This action causes the ice to plow over. This provided a very effective icebreaker as long as the thrust that was supplied by the power plant was sufficient to cause the bow to displace itself under the water, and thus to exert this moment. However, when the bow hits a pressure ridge it can no longer penetrate the ice because it is completely dependent upon the thrust produced by the power plant on the screws.
In boxing, an icebreaker runs its bow onto an ice sheet too thick to be broken by continuous plowing until the ship breaks through the ice at about which time the ship is either at rest in the ice or nearly so; after the ice is at least partially broken, the icebreaker is backed off the ice into the track of broken ice until it is clear of the ice sheet, and again driven to ram into and to ride up onto the ice.
Conventional ice breakers rely upon the mass of the vessel to accomplish breakage of the ice during both continuous plowing and boxing modes of operation. The forward end of an icebreaker may be ballasted to increase the effective portion of the overall mass of the vessel applied to the ice sheet, especially where the vessel becomes stuck on the ice during boxing of very thick ice sheets.
The effectiveness of an icebreaker, measured in terms of the thickness of ice capable of being broken during boxing mode operation, has been determined primarily by the displacement (total weight) of the vessel and by the efficiency with which the specially configured bows of these vessels transferred forward momentum and weight of the vessel downwardly to the ice. The basic objective has been to apply sufficient force downwardly to the ice or by the use of an upwardly acting icebreaker bow structure to cause the ice to break into pieces and to separate from the ice sheet.
The ratio of propulsive horsepower to displacement in icebreakers traditionally has been rather limited considering the task expected of such vessels. Propulsive horsepower has been limited to prevent the vessel from being driven so far up onto a thick ice sheet during boxing mode operation that the vessel cannot be backed off the ice. Bows for icebreakers also are designed to limit hull advance onto an ice sheet to the point where the vessel can be backed off if beached.
Other types of icebreakers have been described in Waas, et al, U.S. Pat. No. 2,902,964 in which weight was transferred cyclically in the vessel to induce pitching and other movements of the hull purportedly in resonance with the corresponding natural periods of the vessel in such movements. The patent describes the use of counterrotating eccentric weights in the vessel, and the shifting of water ballast fore and aft in the vessel; comment is also made therein that water can be pumped into and out of the vessel, all to pitch the vessel, purportedly in resonance with the natural period of the hull.
It is known that icebreakers which are equipped with rotating weight systems may operate to produce cyclic induced motion of the vessel at a frequency of 30 cycles per minute and preferably much greater. Induced pitch experienced at these frequencies by such mechanisms increased the icebreaking efficiency of such vessels. It has been found, for example, that when the induced motion system was operated, the bow of the vessel experienced vertical excursions of 10cm. (total amplitude) at the rate of 30 times a minute.
Thus, it has been discovered that the induced motion frequencies far exceeds the natural motion frequencies of usual icebreaking vessels. Such frequencies had no relation whatever to the natural frequencies of the hulls in question. It has been suggested that much higher frequencies (on the order of 120 cycles per minute) of induced motion would be even more effective. It is highly significant the such induced pitching motions are of small amplitude. It is also known that ballast systems, while effective to produce pitch in a ship under static conditions, cannot be used effectively to produce forces with sufficient rapidity to attain something of a ship's natural rythm of pitch. Thus, prior practical experience with induced hull movements for icebreakers involved high frequency, low amplitude movements resulting from effects internal to the hull.
In a different area pertaining to icebreakers, attempts have been made reliably to predict the areas in which energy is expended by an icebreaker operating during both continuous and boxing modes. It has been estimated that of the total energy expended by an icebreaker in breaking ice, 5% of the energy is consumed in actually breaking the ice, 80% is consumed in moving the ice out of the way of the vessel and in overcoming the buoyancy of the ice, and 15% is consumed in overcoming conventional hull resistance.
Another icebreaker proposal is the so-called RESOD system, based on a combustion explosion. The high pressure exhaust gases from a combustion chamber are emitted under the ice-sheet, and are caused to impinge on the bottom of the ice-sheet. This process has the principal disadvantage that the spacing of the exhaust outlets is exceptionally critical. If the exhaust outlets are spaced at exactly the right point, the energy is received in the ice-sheet and failure does occur. If the exhaust outlets are too close to the ice-sheet, the exhaust literally blows a hole through the ice. If the exhaust outlets are too far beneath the ice-sheet, the energy is absorbed in the water, and there is no fracture in the ice-sheet.
In the Anders Canadian Pat. No. 950,278 issued July 2, 1974 an economical, effective and efficient icebreaker was provided including a pneumatically biased pitch-inducing system which operated at a frequency selected to correspond to the optimum pitch energy transfer characteristic which exists between the vessel, and adjacent ice-sheet, and of broken ice between the ice-sheet and the vessel. Pitching is induced by effectively shifting the center of buoyancy of the vessel, rather than by shifting mass within the vessel. Such prior icebreaker was stated to be efficient in terms of required pitching horsepower because it relied upon the natural dynamic properties of the vessel to accomplish a significant portion of the buoyancy shifting, whereas prior pitch-inducing systems had to work against the dynamic properties of the vessel.
Such icebreakers differed from prior pitching icebreakers by providing low frequency, high amplitude movements of the bow. These high amplitude ship movements involve movement of substantial volumes of water, and water movement was controlled directly and indirectly to assist in moving ice out of the way of the hull in a track formed through an ice sheet. As a result, such icebreaker was able to exert a greater portion of the available power to the ice to be broken and effectively to break greater thicknesses of ice faster than had previously been possible. Such icebreaker operated more efficiently to move ice out of the way of the vessel, thereby significantly reducing what is now the major draim of energy available from the icebreaker.
Also, the bow of such icebreaker was arranged to apply pitching and forward momentum of the vessel to the ice-sheet during both upward and downward movements of the bow. Prior icebreakers operated to apply force only downwardly or upwardly to an ice-sheet.